A resistance bridge is a device for comparing resistors by measuring a ratio between resistors. For instance, a resistance bridge may be used to determine a resistance value of a first resistor that is electrically coupled to a second resistor. The resistance bridge measures electrical parameters of the two resistors, and, based on the electrical measurements, a microprocessor coupled to the resistance bridge calculates a ratio of the two resistors. Resistance bridges may be used in a wide variety of applications that use resistance measurements, such as thermometers, resistor calibrators, multimeters and the like. In general, resistance bridge measurement circuits provide high accuracy measurements. However, the accuracy of any circuit depends on the stability of the electrical components within the circuit. Therefore, the accuracy of a resistance bridge measurement circuit may be limited by the stability of the electrical components within the bridge architecture. For instance, if drift is present in a current source of a resistance bridge measurement circuit, that drift may affect the accuracy of a measurement in the resistance bridge. Although the performance of the current source may be improved, some current drift will remain due to practical limitations in electrical components.
FIG. 1 is a schematic diagram of a resistance bridge measurement circuit 100 in the prior art coupled to two resistors Rs 104 and Rx 106. The resistance bridge measurement circuit 100 includes a current source 102, switches 108, and a measurement circuit 101. The measurement circuit 101 may include an amplifier 110 and an analog to digital converter (ADC) 112. The resistor Rs 104 is a standard or reference resistor and has a known resistance. The resistor Rx 106 may have an unknown resistance, such as a resistor to be calibrated or tested. The reference resistor Rs 104 and the sensor resistor Rx 106 are connected in series. When current is provided to the circuit by the current source 102, current flows through both the reference resistor Rs 104 and the sensor resistor Rx 106 simultaneously. When current flows through each resistor Rs 104 and Rx 106, a voltage across each resistor is generated that is proportional to each resistor's resistance. The amplifier 110 and the ADC 112 measure the voltage across each resistor sequentially. For instance, the voltage across Rs 104 is Vs and the voltage across Rx 106 is Vx. Since only one voltage may be measured at a time, switches 108 are provided to couple the amplifier 110 and ADC 112 between the two resistors Rs 104 and Rx 106. Once the voltage across each resistor Rs 104 and Rx 106 has been measured, the voltages may be converted to a voltage ratio, which correspondence to a resistance ratio:
      Vx    Vs    =      Rx    Rs  
Because the value of the resistor Rs 104 is known, the value of Rx 106 may be determined from the ratio.
Because each resistor Rs 104 and Rx 106 are measured sequentially, any drift in the current source 102 or noise in the circuit may result in an inaccurate measurement. For instance, FIG. 1 illustrates the measurement circuit 100 with the switches 108 coupled to resistors Rs 104. In this state, the amplifier 110 and ADC 112 measures the voltage across the resistor Rs 104. Once the measurement has been made, the switches 108 may change state so that the amplifier 110 and the ADC 112 are coupled to the resistor Rx 106 so that the voltage across the resistor Rx 106 may be measured. Therefore, if the current source drifts between the time at which the resistor Rs 104 is measured and the time at which the resistor Rx 106 is measured, the ratio contains errors that affect the accuracy of the measurement circuit 100. In addition, any noise in the circuit 100 during the measurements may also affect the accuracy of the measurement.
There is, therefore, a need for a more accurate resistance bridge measurement circuit that reduces the affects of noise in the circuit or drift in the current source.